"The Tweets must flow", Twitter declared a year ago, and quickly became an instrument of fast-moving revolution across the Arab world, coordinating mass protests in Egypt and sidestepping the state censorship in Syria. But, the microblogging site conceded that the tweets would not flow evenly in every country.

The company was accused of censorship by many users and threatened with a one-day boycott on Saturday after announcing that it could remove tweets in certain countries which have "different ideas about the contours of freedom of expression".

Twitter insisted that it would not use the gagging system in a blanket fashion, but would apply it on a case-by-case basis, as it happens when governments or organisations complain about individual tweets. But the reassurances were not enough to prevent a torrent of outrage from twitter users and freedom of speech campaigners.

Jeff Jarvis, the media commentator, said the move set the microblogging site onto the "slippery slope of censorship". "I understand why Twitter is doing this – they want to be able to enter more countries and deal with the local laws," he said. "But, as Google learned in China, when you become the agent of the censor, there are problems there."

In a blog on its website, Twitter argued that the change marked an improvement as previously "the only way we could take account of those countries' limits was to remove content globally".

"Starting today, we give ourselves the ability to reactively withhold content from users in a specific country – while keeping it available in the rest of the world," the blogpost said, citing the prohibition of pro-Nazi content in France and Germany. The company also said that any user whose tweets were withheld would be notified, and stressed that Twitter's transparency would be maintained by flagging any withheld tweets on an independent website, Chillings Effects, maintained by the Electronic Frontier Foundation and a group of universities.

The announcement came a day after the first anniversary of the Tahrir Square protests in Egypt, in which Twitter played a prominent role drawing particular criticism from Middle Eastern users. More than half the posts marked with the hashtag #TwitterCensored were in Arabic.

One tweeter from a Gulf Arab state, @abatmeem, parodied the Twitter logo by showing a dead blue bird on its back with feet in the air. "Twitter punctured the silence with its beak, and now it has provoked the tyrants to take revenge," @abatmeem tweeted. "Sorry Twitter bird, you are no longer that bird that could sing all tunes. You have become a parrot that repeats only what is required of it."

Other critical tweets showed the blue bird with a red cross or black strip over its beak. Another Arabic tweeter, @alanoud45 demanded: "How much did they pay you, Twitter?"Twitter insists that the system will only formalise a method it already uses, where tweets are blocked or deleted after full judicial process. Being able to limit tweets to particular countries, rather than blocking them altogether, expands its ability to "let tweets flow".

In theory the system could have been used last year in the UK to block tweets exposing details hidden by superinjunctions about celebrities, or in 2010 when Trafigura used a superinjunction to block the Guardian and BBC from revealing details about a report on activities in Africa. A number of superinjunctions have been abandoned after details leaked on Twitter, to the displeasure of some judges.

Google, Yahoo, eBay and Facebook use similar systems to control what content is shown in which countries.

In China, Google indicates when a search result has been censored. In the same way, blocked tweets will say: "This tweet from [username] is withheld." The blocking can work at the individual tweet or account level.

The US civil liberties website, Demand Progress, opened a petition declaring: "Twitter's importance as an open platform has been demonstrated time and again this year. We need you to keep fighting for and enabling freedom of expression – not rationalize away totalitarianism as a legitimate 'different idea'."

Jillian York, of the Electronic Frontier Foundation, argued that the change was inevitable, given Twitter's global presence. "This is censorship. There's no way around that. But alas, Twitter is not above the law," she wrote.

"Just about every company hosting user-generated content has, at one point or another, gotten an order or government request to take down content," York argued. "Google lays out its orders in its transparency report. Other companies are less forthright. In any case, Twitter has two options in the event of a request: Fail to comply, and risk being blocked by the government in question, or comply [read: censor] And if they have 'boots on the ground', so to speak, in the country in question? No choice."

More from Twitter itself:

It will give users the option to define their country as "worldwide", and that "will show all public tweets" (which would include banned ones).

Twitter spokesperson Jodi Olson said: "We want to reach every person on the planet, and to make Twitter available to people everywhere. The distinction is there are countries which Twitter will not operate in as a business."

Twitter's funding model is to sell adverts against users' tweets, and also to let businesses buy "sponsored" tweets and "trends". By setting up businesses in specific countries, it can sell adverts in those countries for local users. But the service still operates in countries where it does not have its own local operation.

Off message

Twitter is not the first internet giant to control the transmission of content in certain countries.

Yahoo Was sued in 2000 by French civil liberties groups over the sale of Nazi memorabilia via its auction facility. Yahoo had blocked the sale but argued that as it is based in California, Yahoo.com was governed by American law. But US courts ruled they had no jurisdiction in France; the French courts could enforce decisions about Yahoo in their territory.

Twitter Until this week, the entire service could be blocked (as happens in China) or tweets and accounts had to be deleted wholesale, across the world. Now the microblogging service Has a system where tweets and accounts can be blocked in particular countries. It will post them on the Chilling Effects website (which records takedown requests). But observers note that it is giving users clues about who and what has been banned – which could make the original discoverable.

Google Is able to ban content by country: in China it would note when a set of search results had been censored (at the government's order). In Germany and France, searches are filtered.

Facebook Can restrict access to content based on who is viewing it: if it's legal in one country but not in another, Facebook can prevent its viewing in the latter.

eBay In 2000 the auction site changed its policy after public pressure so that Nazi goods and memorabilia cannot not be traded.